A University of California-led research team has discovered 28 new planets deep in the Milky Way, circling stars not unlike our own – leading them to conclude that our solar system may not be so special after all.

“The sun and Earth is not a rarity,” said Geoffrey Marcy, professor of astronomy at UC-Berkeley, estimating that there may be at least 20 million to 30 million solar systems within the Milky Way galaxy. “A family of planets orbiting a single star is a very common occurrence.”

If that is the case, then the likelihood of other Earth-like planets becomes greater.

The newly found planets, reported Monday at the semiannual meeting of the American Astronomical Society in Honolulu, increase to 236 the number of known planets outside our solar system, called “exoplanets.”

Several of these exoplanets are members of multiple-planet systems.

The discovery of these planets suggests that some other solar systems may share some of the same features as ours. Like Venus, Earth and Mars, some planets have rocky interiors and watery surfaces. Close to the star, those planets are warm. Yet others, like Jupiter, Saturn and Uranus, are cold gas giants, distant from the star.

“It is heart-warming that other planetary systems have an architecture that is very reminiscent of our solar system – multiple planets, some with rocky cores and watery envelopes,” Marcy said.

The hope, of course, is that someday an Earth-like planet will be found. Marcy and many other scientists are searching the cosmos for signs of radio or TV signals that would be clues of intelligent life.

But unlike our solar system, most of the new systems have planetary orbits that are elongated, not circular. The resulting vast swings in temperature would not be conducive to a life form that could, for instance, write a piano concerto, build computers or send out signals of its existence, Marcy said.

Indirect evidence

Scientists can’t see a planet directly; rather, they infer its existence from the wobbling and dimming of a star as the planet passes by. But this information can offer enough mathematical detail to create an accurate picture.

For example, the team also described new details about one specific exoplanet, discovered two years ago.

This planet, which circles the star Gliese 436, is thought to be half rock, half water. Its rocky core is surrounded by an amount of water compressed into a solid form at high pressures and low temperatures. It makes a short, 2.6-day orbit around Gliese 436. Based on its radius and density, scientists calculate that it has the mass of 22 Earths, making it slightly larger than Neptune.

The Planet Search team is headed by Marcy; Paul Butler of the Carnegie Institution of Washington; Debra Fischer of San Francisco State University; and Steve Vogt, professor of astronomy at UC-Santa Cruz. They worked with an Anglo-Australian Planet Search team.

The results were reported at the meeting by UC-Berkeley postdoctoral fellow Jason T. Wright and John Asher Johnson, who also has a doctorate.

Number to rise

As detection methods improve, Marcy said, the tally of planets will continue to climb, portraying a universe far vaster and more complex than Galileo ever imagined.

The teams’ Doppler telescope is now sensitive to stellar wobbles of a meter per second, much less than the 10-meter per second limit the groups started out with 15 years ago.

The California and Carnegie Planet Search team uses telescopes at the University of California’s Lick Observatory and the W.M. Keck Observatory in Hawaii.

The Anglo-Australian Planet Search team uses the Anglo-Australian Observatory. Together, these teams have discovered more than half of all known exoplanets.

At Lick Observatory, a new telescope is being built designed specifically to seek Earth-like planets.

The UC-lead team keeps track of all known exoplanets at the Web site of the California and Carnegie Planet Search (http://exoplanets.org), which hosts the only peer-reviewed catalog of information about exoplanets within 652 light-years of Earth.

Lisa M. Krieger is a science writer at The Mercury News, covering research, scientific policy and environmental news from Stanford University, the University of California, NASA-Ames, U.S. Geological Survey and other Bay Area-based research facilities. Lisa also contributes to the Videography team. She graduated from Duke University with a degree in biology. Outside of work, she enjoys photography, backpacking, swimming and bird-watching.

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